1. Field of the Invention:
The present invention relates to radio and sonic systems for ranging and for communication. More particularly, the invention relates to signal processors for reducing interference in such systems from multipath reception or from jamming or from jamming received via multiple paths.
2. Description of the Prior Art:
In radar systems, jamming is a well known countermeasure to prevent effective ranging on a potential target. One or more wideband transmitters for target defense may be provided which are arranged so as to radiate radio frequency energy toward the main radar antenna in the transmission band. In a conventional radar system, the main antenna has a highly directional beam with minor lobes to either side of the main lobe. The interference received via the sidelobes may achieve levels comparable to the main target return and thus interfere with ranging. Devices which attack this problem have been termed "sidelobe cancellers".
Known "sidelobe cancellers" have taken the form of adaptive signal processors in which interference is derived in omnidirectional antennae separate from the main directional antenna. The interference is then reconstructed by correlation techniques in such a way that when subtracted from the main antenna return, the interference is cancelled. In a practical situation, the process is complicated by the possibility of a plurality of jammers and a plurality of transmission paths for each jammer. The known solution has been to create a plurality of correlation feedback loops in which a very complex interference pattern may be reconstructed and then cancelled from the main return. Proposals to achieve this are known and are dealt with in the book entitled "Introduction to Adaptive Arrays" by Monsingo and Miller (John Wily, Chapter 11). The practical complexity of the problem, particularly where a multiplicity of both auxiliary antennae and of time delay values are required, has tended to frustrate standard all-electronic solutions.
The advent of optical processing has provided the possibility of providing a less cumbersome, more versatile and more adaptive solution to the interference problem. In principle, one may consolidate a large number (e.g. 1000) of correlation loops with a 60 MHz bandwidth in a small assemblage of acousto-optical and solid state devices in which the essential correlation processes are carried out in individual pixels of optical patterns, which are transformed and transferred through the assemblage.
While optical processing has been proposed in the literature, neither the practical nor theoretically rigorous requirements for an adaptive optical processing carrier system have been disclosed.
While a major emphasis of investigation in the literature has been directed toward the defense against jamming in radar systems, the principles of interference cancellation are equally applicable to sonar systems as well as to communication systems in which multipath problems interfere with proper signal acquisition.